I'm on 1.17.2. We require a topic before closing a ticket.
At times, tickets need to be merged, and in those cases we indicate the child should be closed. The issue is that if the child does not have a topic assigned, it will stop showing in the list of tickets, but the status in the database remains as "open".
This is not a problem in the UI, but it does affect any reporting that accesses the data directly.